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United States Patent |
5,043,522
|
Leyshon
,   et al.
|
August 27, 1991
|
Production of olefins from a mixture of Cu.sup.+ olefins and paraffins
Abstract
The present invention relates to the conversion of saturated paraffin
hydrocarbons having 4 or more carbon atoms to olefins having fewer carbon
atoms. In particular, the invention provides for contact of a mixture of
40 to 95 wt % paraffin hydrocarbons having 4 or more carbon atoms and 5 to
60 wt % olefins having 4 or more carbon atoms with solid zeolitic catalyst
such as ZSM-5 at conditions effective to form propylene and the separation
of light olefins from the reaction mixture.
Inventors:
|
Leyshon; David W. (West Chester, PA);
Cozzone; Glenn E. (West Chester, PA)
|
Assignee:
|
Arco Chemical Technology, Inc. (Wilmington, DE)
|
Appl. No.:
|
500172 |
Filed:
|
March 27, 1990 |
Current U.S. Class: |
585/651; 208/120.1; 208/120.25; 585/653 |
Intern'l Class: |
C07C 004/02 |
Field of Search: |
585/651,653
208/120
|
References Cited
U.S. Patent Documents
3409701 | Nov., 1968 | Noddings et al. | 585/653.
|
3761538 | Sep., 1973 | Espino et al. | 585/653.
|
4054510 | Oct., 1977 | Parker | 585/653.
|
4247386 | Jan., 1981 | Capierre et al. | 208/61.
|
4251348 | Feb., 1981 | O'Rear et al. | 585/653.
|
Foreign Patent Documents |
276206 | Dec., 1963 | AU | 585/653.
|
0109059 | May., 1984 | EP | 585/653.
|
1911087 | Nov., 1969 | DE | 585/653.
|
233584 | Mar., 1986 | DE | 585/653.
|
178830 | Sep., 1985 | JP | 585/653.
|
0222428 | Nov., 1985 | JP | 585/653.
|
1002705 | Jan., 1986 | JP | 585/653.
|
964918 | Jul., 1964 | GB | 585/653.
|
1397315 | Jun., 1975 | GB | 585/653.
|
Primary Examiner: Garvin; Patrick P.
Assistant Examiner: Fourson; G.
Attorney, Agent or Firm: Long; William C.
Parent Case Text
BACKGROUND OF THE INVENTION
1. Related Applications
This application is a continuation-in-part of copending application Ser.
No. 07/343,097 filed Apr. 25, 1989 now abandoned.
Claims
What is claimed is:
1. The method of preparing C.sub.2 -C.sub.3 olefins from a paraffin
hydrocarbon feedstock which comprises:
a) forming a mixture of 40 to 95 wt % paraffin hydrocarbons having 4 or
more carbon atoms and 5 to 60 wt % olefins having 4 or more carbon atoms,
and feeding said mixture to a reaction zone containing a catalyst
consisting essentially of a zeolite,
b) contacting said mixture with said catalyst at reaction conditions
favoring conversion of said mixed stream to propylene, said conditions
including a reaction temperature in the range 500-700.degree. C., a
hydrocarbon partial pressure in the range of 1 to 30 psia and a paraffin
hydrocarbon conversion per pass of less than 50%, and
c) separating product C.sub.2 -C.sub.3 olefins from the reaction mixture.
2. The method of claim 1 wherein the paraffin hydrocarbon feedstock is a
C.sub.5 to C.sub.20 paraffin hydrocarbon or hydrocarbon mixture.
3. The method of claim 1 wheein the zeolite catalyst is ZSM-5.
4. The method of preparing C.sub.2 -C.sub.3 olefins from a paraffin
hydrocarbon feedstock which comprises:
a) forming a mixture of 40 to 95 wt % paraffin hydrocarbons having 4 or
more carbon atoms and 5 to 60 wt % olefins having 4 or more carbon atoms,
and feeding said mixture to a reaction zone containing a catalyst
consisting essentially of a zeolite,
b) contacting said mixture with said catalyst at reaction conditions
favoring conversion of said mixed stream to propylene, said conditions
including a reaction temperature in the range 500-700.degree. C., a
hydrocarbon partial pressure in the range of 1 to 30 psia and a paraffin
hydrocarbon conversion per pass of less than 50%,
c) separating product C.sub.2 -C.sub.3 olefins from the reaction mixture,
and
d) recycling unreacted paraffin hydrocarbon and olefins formed in step b)
and having 4 or more carbon atoms to step a).
Description
2. FIELD OF THE INVENTION
The present invention relates to the conversion of saturated paraffinic
hydrocarbons to olefins having fewer carbon atoms. In particular, the
invention provides for contact of a mixture of saturated and unsaturated
hydrocarbons comprised of 40% to 95% saturated hydrocarbons with solid
zeolitic catalyst such as ZSM-5 at conditions effective to form propylene.
In preferred practice, light olefins are separated from the reaction
mixture, and unreacted saturated feed and product olefin other than the
desired light olefins product are recycled for further reactive contact
over the zeolite catalyst.
3. Description of the Prior Art
Methods are currently known for the production of commercially important
olefins such as propylene from paraffinic feed materials. Such methods
include steam cracking, propane dehydrogenation, and various refinery
catalytic cracking operations.
Each of these procedures has certain disadvantages. For example, propylene
yields from steam cracking are not very high, and are not substantially
improved by recycling. Purification of non-propylene products is required
which is costly or such products have only fuel value.
Propane dehydrogenation processes ar characterized by rapid catalyst coking
requiring frequent, costly regenerations. Also, reasonable conversions
require sub-atmospheric pressures, and propane is difficult to separate
from propylene.
Propylene supplies from catalytic conversions are uncertain. Transportation
and purification are significant problems.
SUMMARY OF THE INVENTION
The present invention provides an improved process for the selective
production of propylene from C.sub.4 and higher saturated paraffin
hydrocarbon feed, especially C.sub.5 -C.sub.20 paraffin. According to the
invention, the saturated paraffin feed is combined with 5-60 wt. % of
olefins having 4 to 20 carbon atoms and the mixture contacted with a
zeolitic catalyst such as ZSM-5, at conditions which favor propylene
formation, i.e. high temperature and low conversion per pass, and low
hydrocarbon partial pressure. Preferably combined with the saturated feed
hydrocarbon is a recycle stream containing unreacted feed as well as
C.sub.4 + olefins which are formed during the contact with the zeolitic
catalyst and which are not the desired reaction product. Surprisingly,
conditions which favor propylene formation from the saturated paraffin
hydrocarbons also favor propylene formation from butenes and higher
olefins, thus providing enhanced selectivity and yields through practice
of the invention. It has been found that the provision of olefins in the
feed mixture in the designated amounts results in a very substantial
enhancement of saturated hydrocarbon conversion.
DESCRIPTION OF DRAWING
The attached drawing illustrates in schematic fashion preferred practice of
the invention.
DETAILED DESCRIPTION OF THE INVENTION
Although it is known to convert paraffins to lower olefin-containing
mixtures, as above described, prior procedures have not been entirely
satisfactory. Yields via steam cracking are not high. Paraffins can be
converted by reaction over acedic zeolites, but once again yields are not
high.
In accordance with the invention, saturated hydrocarbon conversion to
soluble light olefins can be dramatically improved by incorporated C.sub.4
to C.sub.20 olefins in the feed mixture and passing the resulting mixed
feed over a zeolitic catalyst at conditions favoring propylene formation.
Saturated hydrocarbons employed as feed are paraffins having at least four
carbon atoms and are preferably C.sub.5 to C.sub.20 paraffins. It is
essential that the feed mixture to the conversion zone contain between 40
and 95 wt. % of these paraffins based on the total of paraffins and
olefins for the advantages of the present invention to be realized.
Combined with the paraffins in the conversion feed mixture are C.sub.4 to
C.sub.20 olefins in amount of 5 to 60 wt. % based on the total of
paraffins and olefins, preferably 10 to 50 wt. % olefins.
The feed mixture may also contain aromatics, naphthenes and inerts such as
nitrogen, but the benzene content should not exceed 30 wt. % of the total
feed. At benzene concentrations above 40 wt. %, alkylation becomes
significant and light olefin yields are reduced. The feed mixture may also
contain steam in amount up to 30 mol. %, preferably 1 to 20 mol. %.
The accompanying drawing illustrates a particularly preferred practice of
the invention involving recycle of C.sub.4 and higher olefins formed
during the paraffin conversion.
Referring to the drawing, the paraffin hydrocarbon feedstock, e.g., C.sub.6
-C.sub.20 paraffin hydrocarbons, passes via line 1 to reaction zone 2.
Recycle comprised of unreacted paraffins and C.sub.4.sup.+ olefins passes
via line 3 and is combined with the net paraffin feed to form a mixture of
40 to 95 wt. % paraffins, and this mixture is fed to zone 2. In zone 2 the
mixed hydrocarbon feedstock is contacted with the zeolite solid contact
catalyst at reaction conditions which favor production of propylene from
both the paraffin and olefin feed materials.
Conditions favoring propylene production involve low hydrocarbon partial
pressure, high temperatures and low per pass conversions as described
later.
The product mixture from reaction zone 2 passes via line 4 to separation
zone 5 wherein the components of the product mixture are separated by
conventional means such as fractional distillation.
An overhead mixed ethylene and propylene stream is removed from zone 5 via
line 6 and comprises the preferred product mixture. Higher boiling
compounds are removed via line 7; a small purge of hydrocarbons suitable
as gasoline blending stock is separated as by distillation (not shown) via
line 8 with the remainder of the materials boiling higher than propylene
being recycled via line 3 for further reaction in zone 2 after being
combined with the fresh paraffin feed introduced via line 1.
In order to more clearly describe the invention particularly in comparison
to procedures which are not in accordance with the invention, reference is
made to the following examples.
COMPARATIVE EXAMPLE A
H ZSM-5, 20.times.40 mesh, in amount of 0.25 grams was admixed with 3.5
grams of similar mesh size alpha alumina and loaded into a 36 inch tubular
reactor made from 5/8 inch OD tubing having 0.065 inch wall thickness.
Reactor heating was by an electric tube furnace.
Normal octane was fed to the top of the reactor where it was preheated to
510.degree. C. before contacting the catalyst. Conditions in the catalyst
bed were maintained at about 527.degree. C. and 6 psig. Octane feed rate
was 250 cc/hr giving a WHSV of about 700 hr.sup.-1 based on the ZSM-5.
Residence time was about 0.1 second.
The reactor was operated for 1 hour between regenerations. Regeneration
consisted of feeding 5% O.sub.2 for 28 minutes and full air for 28 minutes
followed by 4 minutes of nitrogen purge.
Octane conversion was 22%, and the wt. % selectivities achieved on a
hydrogen-free and coke-free basis were:
______________________________________
Methane 0.4
Ethylene
4.59
Ethane 2.03
Propylene
15.55
Propane 9.95
Butenes 21.89
Butanes 16.14
Pentenes
9.77
Pentanes
6.36
C.sub.6 's
3.46
C.sub.7 's
2.93
C.sub.9.sup.+
6.94
______________________________________
EXAMPLE 1
Comparative Example A was repeated except that, pursuant to the instant
invention, an equal volume of the C.sub.4.sup.+ reaction products
(containing unreacted octane and olefins) was continuously recycled to the
reaction and combined wth the fresh octane feed. The combined feed
contained 2.4 wt. % propylene and 7.2 wt. % C.sub.4 plus C.sub.5 olefins.
The n-octane concentration in the combined feed was 85 wt. %. Residence
time was 0.05 second. The WHSV including the recycle was about 1400
hr.sup.-1.
Overall octane conversion was 31% and overall wt. % selectivities on a
hydrogen-free and coke-free basis were:
______________________________________
Methane 0.59
Ethylene
6.32
Ethane 2.55
Propylene
27.88
Propane 14.90
Butenes 21.34
Butanes 6.86
Pentenes
5.66
Pentanes
4.70
C.sub.6 's
2.28
C.sub.7 's
2.06
C.sub.9.sup.+
3.96
______________________________________
It will be seen from a comparison of Example 1 and Comparative Example A
that practice of the invention dramatically improved overall octane
conversion as well as overall propylene selectivity. The butane and
pentane selectivities were sharply reduced.
COMPARATIVE EXAMPLE B
Comparative Example A was repeated except that the feed was cis/trans
butene-2 rather than n-octane.
Butene conversion was 65% and the wt. % selectivities on a hydrogen-free
and coke-free basis were:
______________________________________
Methane 0.05
Ethylene
4.26
Ethane 0.12
Propylene
29.55
Propane 3.20
Butanes 6.56
C.sub.5.sup.+
56.26
______________________________________
this Comparative Example B demonstrates about the same selectivity to
propylene as Example 1 and illustrates that surprisingly both saturated
hydrocarbons and olefins are converted to propylene with good efficiency
at the same reaction conditions.
COMPARATIVE EXAMPLE C
A full range naptha (C.sub.5 to C.sub.12), condensed from an Algerian
natural gas well, was fed to a steam cracker at an 0.75 steam to oil
ratio. The weight % yields are shown below at the indicated coil
temperatures:
______________________________________
Coil Outlet Temp, .degree.C.
815 849
______________________________________
Yields, wt %
Hydrogen 0.8 1.0
Methane 12.8 16.5
Acetylene 0.3 0.6
Ethylene 25.0 30.0
Ethane 3.7 3.4
Methylacetylene/Propadiene
0.6 0.7
Propylene 16.9 12.1
Propane 0.7 0.5
Butenes 5.7 3.2
Butanes 1.1 0.4
Butadiene 4.6 3.9
C.sub.5 's 5.4 3.2
Benzene 6.6 9.1
Toluene 3.7 4.1
C.sub.8 Aromatics 2.5 2.7
C.sub.6 to C.sub.8 Non-aromatics
4.1 1.0
C.sub.9.sup.+ 5.5 7.5
______________________________________
COMPARATIVE EXAMPLE D
0.15 grams of ZSM-5 catalyst, 100.times.140 mesh was mixed with 4.5 grams
of Alcoa T-64 alpha alumina and loaded into the reactor of Example A. The
reactor was heated with an electric tube furnace. Temperatures in the
catalyst bed were measured with an axial thermowell. Algerian condensate
as in Example C was pumped into the top of the reactor at the rate of 60
cc/hr. The catalyst bed was maintained at 621.degree. C. and 0 5 psig. The
gas and liquid reaction products were analyzed and the results are shown
below:
______________________________________
Overall conversion, %
54
______________________________________
Yield, Wt %
Methane 3.8
Ethylene 9.8
Ethane 3.5
Propylene 18.3
Propane 2.3
Butenes 8.2
Butane 4.3
Butadiene 0.2
C.sub.5 olefins 2.5
C.sub.5 paraffins 1.1
C.sub.6.sup.+ aromatics
3.3
C.sub.6.sup.+ paraffins (unreacted feed)
42.7
______________________________________
EXAMPLE 2
The process of the present invention was carried out using the Algerian
condensate also used in Comparative Examples C and D. Conditions of the
reaction were the same as those in Comparative Example D. In carrying out
the process, effluent from the reactor was distilled to separate a C.sub.3
and lighter product stream from a C.sub.4 and heavier stream which was
recycled. The volume ratio of fresh feed to recycle was varied. The
results achieved are as follows:
______________________________________
Recycle Ratio
Recycle/Fresh Feed
1.0 2.0 4.0
______________________________________
Overall Yields, wt %
(based on fresh feed)
Methane 6.1 6.7 7.0
Ethylene 16.3 18.2 19.2
Ethane 5.6 6.1 6.4
Propylene 32.9 37.5 40.1
Propane 3.9 4.3 4.6
Butanes 7.3 8.2 8.8
Butenes 4.6 2.5 1.2
C.sub.5.sup.+ 23.3 16.5 12.7
______________________________________
As shown in Comparative Example C, steam cracking is capable of a 17%
propylene yield and this will not increase beyond 20% with recycle because
the once through C.sub.4.sup.+ cracking products are not well suited for
making propylene. Through practice of the present invention, propylene
yields as high as 40% can be achieved thus demonstrating the surprising
superiority of the invention.
COMPARATIVE EXMAPLE E
10.0 grams of Intercat Zcat-plus, 40.times.60 mesh, was loaded into a 3/4"
ID Alumina tube. The catalyst bed was supported with a 1/2" OD alumina
tube from the bottom. A layer of Denstone 57 inert spheres was placed on
top. The ceramic tube was placed inside a 1.8" OD stainless steel shield.
The entire assembly was mounted in an electric tube furnace. 93 gm/hr of
n-butane and 6 gm/hr of distilled water were fed to the catalyst bed,
which was maintained at about 593.degree. C. and 1 psig. After 30 minutes
the butane feed was stopped and the catalyst bed was regenerated using
air, steam and nitrogen. The regeneration was conducted for 30 minutes
also. Following this, the reaction-regeneration cycle would repeat
indefinitely, until steady state was searched. When steady state was
reached, the following data were obtained:
______________________________________
Conversion of N-butane, %
7.2 10.9
Selectivity, wt %
Methane 14.05 15.90
Ethylene 15.66 19.14
Ethane 12.51 10.23
Propylene 38.35 40.72
Propane 1.21 1.62
Butanes 16.12 9.54
Isobutane 0.80 0.24
Pentenes 0.0 0.93
Pentanes 0.0 0.0
C.sub.6.sup.+ 0.25 0.79
______________________________________
COMPARATIVE EXAMPLE F
The procedure of Example E above was repeated, except the temperature was
raised to 635.degree. C. and the feed was changed to isobutane.
______________________________________
Conversion of Isobutane, %
16.1
Selectivity, wt %
Methane 12.6
Ethylene 3.7
Ethane 0.26
Propylene 34.13
Propane 1.95
n-butane 1.38
Butenes 39.21
C.sub.5.sup.+ 6.77
______________________________________
EXAMPLE 3
Using the same procedure as Example E, gasoline hydrocarbon mixture was fed
to the catalyst bed at a rate of 108 grams per hour. The temperature and
pressure in the catalyst bed were maintained at 593.degree. C. and 2 psig,
respectively. The feed, analyzed by FIA and GC had the following
composition:
______________________________________
Olefins 10.5 vol % Isobutane 3.6 Wt %
Saturates 61.0% N-butane 5.6%
Aromatics 28.5% Pentanes 15.3%
______________________________________
At steady state, the overall conversion was 20.3%. The component
selectivities and conversions are presented below:
______________________________________
Component
Selectivity, wt %
Conversion
______________________________________
H2 + Coke 1.44
Methane 3.68
Ethylene 16.20
Ethane 2.97
Propylene 42.02
Propane 2.67
Isobutane 32.3
BD 0.24
N-butane 21.9
Butenes 12.32
Pentenes 62.4
Pentanes 19.4
C.sub.6 's 33.2
Benzene 5.44
C.sub.7 's 36.7
Toluene 7.23
C.sub.8 Non-Aromatics 21.7
C.sub.8 Aromatics
4.28
C.sub.9.sup.+ 1.52
______________________________________
Comparing Example 3 with Examples E and F indicates that the conversions of
N-butane and isobutane are doubled in the presence of olefins.
EXAMPLE 4
A C.sub.4 -C.sub.6 cut was taken from an FCC unit and reacted using the
same procedure as Example E. The olefin concentration in this stream was
55 wt %. The balance was paraffins, including 17.1 wt % isobutane and 7.6
wt % n-butane. 100 gm/hr of this mixture was fed to the catalyst bed,
which was held at 593.degree. C. and 1 psig. The results are summarized
below:
______________________________________
Overall Conversion: 34%
Component
Selectivity, wt %
Conversion, %
______________________________________
Methane 1.51
Ethylene 16.04
Ethane 0.60
Propylene 53.22
Propane 2.10
Isobutane 25.5
BD 59.3
N-butane 3.2
Butenes 46.0
Pentenes 59.2
Pentanes 14.36
C.sub.6 's 35.0
benzene 1.84
C.sub.7 's 2.86
Toluene 4.10
C.sub.8.sup.+
3.18
______________________________________
The isobutane conversion here is nerely double that of Example E, in spite
of the lower temperature and lower isobutane concentration. This suggests
olefins in the feed are increasing the rate of paraffin consumption.
COMPARATIVE EXAMPLE G
BT Raffinate, containing the components listed below, was reacted according
to the procedure of Example E at feed rate of 210 gm/hr:
______________________________________
Feed, wt %
______________________________________
Butenes
0.19
Pentenes
0.21
Pentanes
5.91
Hexanes
65.88
Benzene
1.51
Hextanes
19.20
Toluene
2.10
C.sub.8.sup.+
3.00
______________________________________
The total olefin content of this stream is 0.4 wt %. The catalyst bed was
maintained at 538.degree. C. and 9 psig. The results are shown below:
______________________________________
Component
Selectivity, wt %
Conversion, %
______________________________________
Coke + H.sub.2
1.25
Methane 2.82
Ethylene 7.84
Ethane 3.35
Propylene 38.05
Propane 4.45
Butenes 28.65
Butanes 1.40
Pentenes 7.80
Pentanes 2.99
Hexanes 15.04
Benzene 1.53
Hextanes 18.73
Toluene 1.71
C.sub.8.sup.+
2.87
______________________________________
The relative conversions of C.sub.5, C.sub.6 and C.sub.7 paraffins are not
surprising, since reactivity increases with molecular weight.
EXAMPLE 5
According to the ivnetnion, a C.sub.4 -C.sub.6 cut containing 55 wt %
olefins was taken from an FCC unit and fed according to the procedure of
Example E at the rate of 195 gm/hr. The composition of the stream is
presented below:
______________________________________
Feed, wt %
______________________________________
Propylene 0.10
Propane 0.14
Isobutane 17.11
N-butane 7.60
Butenes 39.23
Pentenes 16.36
Pentanes 13.80
C.sub.6.sup.+ 5.66
______________________________________
The catalyst bed was maintained at 538.degree. C. and 9 psig. The C.sub.3
and lighter portion of the reactor effluent was separated by continuous
fractionation and removed as product. The C.sub.4.sup.+ portion of the
effluent was recycled back to the reactor inlet at a rate of 256 gm/hr and
mixed with the fresh feed prior to contact with the catalyst bed, bringing
the total feed rate to 451 gm/hr. The composition of the combined feed is
shown below:
______________________________________
Feed, wt. %
______________________________________
Propylene 1.61
Propane 0.06
Isobutane 13.25
N-butane 7.59
Butenes 26.39
Pentenes 21.49
Pentanes 15.27
C.sub.6.sup.+ 14.34
______________________________________
The total concentration of olefins in the reactor feed was 49.5 wt %.
The overall conversion of the fresh feed was 32%. The overall selectivities
and component conversions are shown below:
______________________________________
Overall Selectivity, wt %
Overall Conversion %
______________________________________
Methane 0.26
Ethylene
11.05
Ethane 0.23
Propylene
75.09
Propane 1.83
Isobutane 6.35
N-butane 3.65
Butenes 56.03
Pentenes 26.53
Pentanes 29.56
C.sub.6.sup.+
11.43
______________________________________
Comparing Exmaple G with Example 5 shows the increase in C5 paraffin
conversion resulting from the presence of olefins in the feed. It is
surprising that the C.sub.5 paraffin conversion in Example 5 is ten times
higher than Example G, in spite of the fact that the space velocity is
twice as high. This result shows the beneficial effect of olefins on the
conversion of paraffins.
EXAMPLE H
The procedure of Example E was repeated, except the feed was isobutane, the
temperature was raised to 635.degree. C. and the pressure was raised to 12
psig.
______________________________________
Conversion of Isobutane, %
23.7
Selectivity, wt %
Methane 12.47
Ethylene 8.03
Ethane 0.62
Propylene 35.83
Propane 4.26
N-butane 1.77
Butenes 29.02
C.sub.5.sup.+ 5.96
______________________________________
EXAMPLE 6
Example 4 was repeated, except at 12 psig pressure. The overall conversion
was 42.3%.
______________________________________
Component
Selectivity, wt %
Conversion, %
______________________________________
Methane 7.9
Ethylene 18.8
Ethane 2.6
Propylene 51.4
Propane 4.6
Isobutane 33.1
BD 57.8
N-butane 13.5
Butenes 61.8
Pentenes 65.7
Pentanes 2.8
C.sub.6.sup.+
11.4
______________________________________
A comparison of Example 6 with example H shows that the isobutane
conversion in Example 6 is measurably higher even though the temperature
was 24.degree. C. lower. This is due to the olefins present in the feed to
Example 6.
Saturated paraffin hydrocarbons used as feed in accordance with the
ivnention are those having 4 or more carbon atoms, especialy C.sub.5
-C.sub.20. Individual hydrocarbons or mixtures can be employed. Preferred
hydrocarbons are those having from about 6 to 20 carbons, especially
petroleum fractions for reasons of costs. Specific hydrocarbons include
hexane, the methyl pentanes, cetane, etc.
The conversion is carried out at elevated temperatures in the range of
about 400.degree. to 800.degree. C., preferably 500.degree. to 700.degree.
C.
Low hydrocarbon partial pressures and low conversions per pass favor
propylene production. The feed hydrocarbon can be admixed with steam or
inert gas such as nitrogen. The hydrocarbon partial pressure is as low as
practical, illustratively 1 to 30 psia. Where no diluents are employed,
system pressures ranging from about -12 to 50 psig, preferably -5 to 30
psig are suitable. Higher pressures can be used when diluents are
employed.
High space velocity and short residence times are preferred in order to
maintain the desired low conversions per pass. Paraffin hydrocarbon
conversions per pass are less than 50%. Space velocities depend on the
particular zeolite used and are 1 to 5000 preferably 5 to 2000 hr.sup.-1
WHSV. Reactor residence times are 0.001 to 20 seconds, preferably 0.01 to
5 seconds.
The conversion reaction of the instant invention is highly endothermic.
Preferably fluidized solid catalyst conversion procedures are used with
the feed hydrocarbon vapor contacting fluidized particles of the zeolite
catalyst Heat necessary to maintain the reaction is provided by separately
heating the catalyst particles in a fluidized regeneration zone as by
combustion of appropriate fuel hydrocarbon.
Fixed bed procedures can be employed. In such cases, the use of reaction
zones in series with interstage heating is advantageous.
Zeolite catalysts used in the invention can be silaceous, crystalline
molecular sieves. Such silica containing crystalline materials include
materials which contain, in addition to silica, significant amounts of
alumina. These crystalline materials are frequently named "zeolites, i.e.,
crystalline aluminosilicates. Silica containing crystalline materials also
include essentially aluminum-free silicates. These crystalline materials
are exemplified by crystalline silica polymorphs (e.g., silicalite,
disclosed in U.S. Pat. No. 4,061,724 and organosilicates, disclosed in
U.S. Pat. No. Re. 29948), chromia silicates (e.g., CZM), ferrosilicates
and galliosilicates (see U.S. Pat. No. 4,238,318), and borosilicates (see
U.S. Pat. Nos. 4,226,420; 4,269,813; and 4,327,236).
Crystalline aluminosilicate zeolites are best exemplified by ZSM-5 (see
U.S. Pat. Nos. 3,702,886 and 3,770,614), ZSM-11 (see U.S. Pat. No.
3,709,979), ZSM-12 (see U.S. Pat. No. 3,832,449), ZSM-21 and ZSM-38 (see
U.S. Pat. No. 3,948,758), ZSM-23 (see U.S. Pat. No. 4,076,842), and ZSM-35
(see U.S. Pat. No. 4,016,246).
Phosphorous containing zeolites are suitably used (see U.S. Pat. 3,972,832)
and in such cases it is especially advantageous to add steam to the feed
mixture.
Acid aeolites are especially preferred, particularly the ZSM type and
borosilicates. ZSM-5 is especially useful.
In addition to the above, zeolite containing materials can be used.
Representative of such materials are zeolite A (U.S. Pat. No. 2,882,243),
zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007),
zeolite ZK-5 (U.S. Pat. No. 3,247,195), zeolite ZK-4 (U.S. Pat. No.
3,314,752), synthetic mordenite, and dealuminized mordenite, as well as
naturally occurring zeolites, including chabazite, faujasite, mordenite,
and the like.
In general, the zeolites are ordinarily ion exchanged with a desired cation
to replace alkali metal present in the zeolite as found naturally or as
synthetically prepared. The exchange treatment is such as to reduce the
alkali metal content of the final catalyst to less than about 1.5 weight
percent, and preferably less than about 0.5 weight percent. Preferred
exchanging cations are hydrogen, ammonium, rare earth metals and mixtures
thereof, with particular preference being accorded rare earth metals. Ion
exchange is suitably accomplished by conventiona contact of the zeolite
with a suitable salt solution of the desired cation, such as, for example,
the sulfate, chloride or nitrate salts.
It is preferred to have the crystalline zeolite of a suitable matrix, since
the catalyst form is generally characterized by a high resistance to
attrition, high activity and exceptional steam stability. Such catalysts
are readily prepared by dispersing the crystalline zeolite in a suitable
siliceous sol and gelling the sol by various means. The inorganic oxide
which serves as the matrix in which the above crystalline zeolite is
distributed includes silica gel or a cogel of silica and a suitable metal
oxide Representative cogels include silica-aluminia, silica-magnesia,
silica-zirconia, silica-thoria, silica-beryllia, silica-titania, as well
as ternary combinations, such as silica-alumina-magnesia,
silica-aluminia-zirconia and silica-magnesia-zirconia. Preferred cogels
include silica-alumina, silica-zirconia or silica-alumina-zirconia. The
above gels and cogels will generally comprise a major proportion of silica
and a minor proportion of the other aforementioned oxide or oxides. Thus,
the silica content of the siliceous gel or cogel matrix will generally
fall within the range of 55 to 100 weight percent, preferably 60 to 95
weight percent, and the other metal oxide or oxides content will generally
be within the range of 0 to 45 weight percent, and preferably 5 to 40
weight percent. In addition to the above, the matrix may also comprise
natural or synthetic clays, such as kaolin type clays, montmorillonite,
bentonite or halloysite. These clays may be used either alone or in
combination with silica or any of the above specified cogels in a matrix
formulation.
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